Electrode for forming a cooling channel in a wall

ABSTRACT

A wall in which there is formed at least one cooling channel, said wall being cooled by cool air flowing in the channel, the channel comprising a hole and a diffusion portion, the hole opening out at one end into the inside surface of the wall, and at its other end into the diffusion portion where it forms an orifice, the diffusion portion flaring around said orifice and opening out into the outside surface of the wall, the diffusion portion having a bottom whose front end is substantially plane, sloping, and extending in front of the orifice, and also having a margin extending behind, round the sides, and in front of the orifice, said margin joining the sides of the front end. A method and an electrode for making such a cooling channel. A turbomachine blade presenting such a wall.

This application is a divisional of U.S. application Ser. No. 12/787,477filed May 26, 2010, which is a divisional of U.S. application Ser. No.11/492,110 filed Jul. 25, 2006, the entire contents of each of which areincorporated herein by reference. U.S. application Ser. No. 11/492,110claims the benefit of priority under 35 U.S.C. 119 to French ApplicationNo. 0507924 filed Jul. 26, 2005 and French Application No. 0650103 filedJan. 12, 2006.

The invention relates to a method of forming a cooling channel in awall, to an electrode used for implementing the method, to a wallelement in which a cooling channel is formed, and to a hollowturbomachine blade including a wall element of this type.

FIELD OF THE INVENTION

More precisely, the invention relates to a wall element of the typecomprising an inside surface and an outside surface, the outside surfacebeing suitable for being cooled by cool air flowing in said coolingchannel. In addition, the cooling channel is of the type comprising ahole and a diffusion portion, the hole opening out at one end into theinside surface of the wall and at its other end substantially into thebottom of the diffusion portion by forming an orifice, and the diffusionportion flaring away from said orifice and opening out into the outsidesurface of the wall.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 6,183,199 B1 shows an example of a wall element for ahollow blade of a turbojet turbine, the element being pierced by acooling channel of the above-specified type. In that example, the holeof the channel and its diffusion portion are made by electroerosion in asingle step, using a single electrode whose tip presents a front portionof a shape that corresponds to that of the hole and a rear portion of ashape corresponding to that of the diffusion portion of the channel.

An electrode of that type is described and shown in U.S. Pat. No.4,197,443 to which reference is made in U.S. Pat. No. 6,183,199 B1. Ascan be seen, the shape of that electrode is particularly complex. Inaddition, in general manner, forming a channel by electroerosion usingknown methods remains an operation that is lengthy and expensive.

OBJECTS AND SUMMARY OF THE INVENTION

An object of the invention is thus to provide an alternative to knownmethods, making it possible to form a cooling channel of theabove-specified type, more quickly and at lower cost.

To achieve this object, the invention provides a method wherein saidwall is pierced to make said hole, and wherein an indentation is formedin said wall in order to form said diffusion portion, in two distinctsteps.

In the method of the invention, it is thus possible to use techniquesand materials that are different for making the hole and for making thediffusion portion.

To make the hole, it is possible to pierce the wall by electroerosion orby using a laser.

Advantageously, laser piercing techniques are used, since thesetechniques are much faster and much less expensive than electroerosiontechniques. Thus, to pierce a wall of a hollow turbine blade by laser,generally only a few tenths of a second are required.

Nevertheless, if it is desired to perform the piercing and the diffusionportion by electroerosion, then two different electrodes are used foreach of said portions. The electrodes are simpler in shape than theelectrodes that have been used in the past, so they are easier and lessexpensive to make. For example, for the piercing, it is possible to usean electrode that is cylindrical.

According to another aspect of the invention, in order to make thediffusion portion, an electrode is used in which the tip of theelectrode is in the form of a cone with a rounded end and with a conicalside surface that presents a flat, the axis of the cone not intersectingthe flat.

The above-specified shape for the electrode makes it possible firstly toavoid creating a sharp angle in the bottom of the diffusion portion,where sharp angles generally constitute starter zones for cracks.

Secondly, an electrode of that shape and of appropriate dimensionsrelative to the hole makes it possible to form a diffusion portion of ashape that is sufficiently broad and flared to ensure that, given thetolerances involved in making the hole and the diffusion portion, andregardless of the position of the hole relative to the diffusionportion, the diffusion portion provides good diffusion (i.e. goodguidance and good spreading) of the stream of air leaving the hole.

The invention also provides a wall element having a new type of coolingchannel formed therein. Such a channel may be obtained using the methodand the electrode of the invention.

The channel includes a diffusion portion with a bottom whose front endis substantially flat, sloping in the thickness of the wall andextending in front of the orifice in the flow direction of the cool air,and a rearwardly-extending margin on the sides and at the front of theorifice, said margin joining the sides of the front end of the bottom.

Advantageously, the angle formed between the margin and the front end ofthe bottom in a plane perpendicular to the bottom is not “sharp” in thesense that it is strictly greater than 90°. This avoids creatingcrack-starter zones.

Finally, the invention provides a hollow gas-turbine blade including awall element of the above-specified type.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its advantages can be better understood on reading thefollowing detailed description. The description refers to theaccompanying figures, in which:

FIG. 1 is a section showing an example of a wall element of theinvention including a cooling channel;

FIG. 2 is a perspective view of the tip of the electrode used for makingthe diffusion portion of the FIG. 1 channel;

FIG. 3 is a plan view of the FIG. 1 channel seen looking along thedirection III that is orthogonal to the outside surface of the wall;

FIG. 4 is a view of the FIG. 1 channel seen looking along the directionIV, i.e. along the piercing axis of the channel; and

FIG. 5 is a section view on plane V-V of FIG. 3.

MORE DETAILED DESCRIPTION

With reference to FIGS. 1, 3, 4, and 5, there follows a description ofan example of a wall element of the invention.

Said wall element presents an inside surface 3 and an outside surface 5.This element belongs to a wall 1 of a hollow gas-turbine blade, such asa high pressure turbine blade of a turbojet. This type of hollow bladehas an internal cooling passage 4 defined in part by the inside surface3, said passage being fed with cool air.

The outside surface 5 of the wall is subjected to the hot gas passingthrough the turbine and it therefore needs to be cooled. For thispurpose, cooling channels are provided in the wall 1. At least some ofthese channels are of the same type as the channel shown in FIG. 1. Thischannel 6 passes cool air coming from the internal cooling passage 4 ofthe blade and delivers this cool air to the outside surface 5 in orderto cool it. The channel 6 comprises two portions: an adjustment portionformed by a hole 7, and a diffusion portion 9 formed by an indentationformed in the wall 1 in its outside surface 5.

The hole is said to be an adjustment portion since the minimum sectionof the hole 7 serves to adjust the rate at which air flows along thechannel 6. Advantageously, the hole 7 is simple in shape. In the exampleshown, the hole 7 is inscribed in a circular cylinder. In addition, theaxis B of the hole 7 is inclined at an angle G relative to the outsidesurface 5 (or if this surface 5 is not plane, relative to the tangentthereto on the axis B). The angle G is less than 90°, and preferablylies in the range 15° to 80°, so as to direct the stream of air Ftowards the outside surface 5 so that it remains as close as possiblethereto. In other words, it is desired to make the velocity vectors ofthe air stream F at the outlet from the channel 6 as nearly parallel aspossible to the plane of the outside surface 5.

In order to direct the stream of air F better against the outsidesurface 5 and in order to spread this stream of air F in the plane ofthe outside surface 5, the channel 6 presents a diffusion portion 9following the hole 7. This diffusion portion 9 flares around the orifice11 through which the cool air leaves the hole 7. This orifice 11 issituated preferably substantially in the bottom of the diffusion portion9 relative to the outside surface 5. In front of the orifice, in theflow direction of the stream F, the diffusion portion 9 has a bottomwhose front end 13 is substantially plane, sloping in the thickness ofthe wall at an angle g relative to the outside surface 5. The angle gpreferably lies in the range 2° to 45°, and in any event it is smallerthan the angle G so that the stream of air F, as guided by the front end13 of the bottom is directed towards the outside surface 5.

The front end 13 of the bottom encourages the stream of cool air Fleaving the hole 7 to come close to the outside surface 5. This airstream thus remains in contact with the outside surface 5, thus makingit possible firstly to cool the surface 5 by heat exchange and secondlyto create a protective film of air on said surface 5 that keeps the hotgas of the medium in which the wall 1 is situated at a distance fromsaid surface 5.

Advantageously, the outline of the front end 13 of the bottom isgenerally triangular in shape, having one of its vertices pointingtowards said orifice 11 (see FIGS. 3 and 4), thus enabling the airstream F leaving the hole 7 to be spread, and thus cooling andprotecting a larger portion of the outside surface 5. Naturally, thebase remote from said vertex is wider than the orifice 11, so as towiden the air stream F.

At the rear end, on the sides and in front of the orifice 11, there is amargin 15. The margin 15 goes around the orifice 11 in part and towardsthe front it joins the sides of the front end 13 of the bottom.

In the example shown in FIG. 5, the junction zones between the margin 15and the front end 13 present edges 17. The angle P formed in these edgesbetween the margin 15 and the front end 13 itself, in a planeperpendicular to the front end 13, is strictly greater than 90° so as toavoid weakening the wall element 1. The angle P is measured between thetangent T to the margin 15 at the edge 17, and the front end 13 of thebottom, as shown in FIG. 5.

It is also possible to provide rounding in each junction zone in orderto avoid creating any edges. Under such circumstances, the angle P ismeasured between the general direction of the margin 15 and the frontend 13 of the bottom.

In the example of FIG. 1, the rear portion of the margin 15 flaresrearwards from the orifice 11 and then presents a forwardly-directed lip12 in the outside surface 5. This lip 12 helps guide the stream of coolair forwards.

The front end 13 of the bottom and the margin 15 are inscribed in a cone23 having a rounded end 24 and a conical surface 25 that presents a flat26. The front end 13 of the bottom corresponds to the flat 26, and ascan be seen in FIGS. 1 and 4, the margin 15 corresponds essentially inits rear portion with the rounded end 24 of the cone 23, and in its sideand front portions with the conical surface portions 25 adjacent to theflat 26. Advantageously, the axis E of the cone 23 and the axis B of thehole 7 are parallel, with the axis B preferably being offset towards theoutside surface 5, as shown in FIG. 1.

With the shape of the cooling channel 6 in the wall element 1 describedfully above, there follows a description of an implementation of themethod of the invention enabling a channel of this type to be formed.

In a first step of the method, the wall 1 is pierced by means of alaser. Laser piercing techniques are known to the person skilled in theart and they present the advantage of being fast and less expensive thanelectroerosion techniques.

Then, in a second step, the recess corresponding to the diffusionportion 9 is formed in the wall 1 in its outside surface 5 byelectroerosion. Naturally, this second step could be undertaken beforethe first.

For this second step, an electrode 20 is used that is of the type shownin FIG. 2. The body of the electrode 21 is cylindrical while the tip 22of the electrode is in the form of a cone 23 having a rounded end 24 andincluding a flat 26 in its lateral conical surface 25. The flat 26extends on one side of the cone 23 from a point in the vicinity of theend 24 to the portion of the cone 23 where it is at its greatest flare,and beyond. The axis E of the cone 23 does not intersect the flat 26:the flat therefore does not intersect the end vertex 24 of the cone 23.The cone 23 is symmetrical about a plane of symmetry S perpendicular tothe flat 26 and containing the axis E of the cone 23. The half-angles Yof the flare defined between the side edges 27 of the flat and the planeS lie in the range 10° to 30°, and are preferably close to 15°.

As shown in FIG. 1, the indentation corresponding to the diffusionportion 9 is formed by electroerosion, by pushing the tip 22 of theelectrode 20 into the wall 1 via its outside surface 5, the flat 26being positioned to face said outside surface 5. Advantageously, duringthis operation, the axis E of the cone 23 is oriented so as to beparallel to the axis B of the hole 7, with these axes preferably beingoffset so that the axis B is the closer to the wall 1.

It should be observed that the presence and the size of the lip 12 ofthe margin 15 (i.e. the extent to which it extends rearwards) depends onthe radius of the curvature of the end 24 and on the depth to which theelectrode 20 is pushed in.

In general, when forming the indentation, the following are selected:the shape of the electrode 21 and more particularly the shape of thecone 23, the radius of the curvature of the rounded end 24 and theposition of the flat 26 (half-angles Y of the flare); the position ofthe electrode, and more particularly the orientation of the axis E ofthe cone 23 relative to the axis B of the hole 7; and the penetrationdepth of the electrode 20 into the wall 1; so as to form the front end13 of the bottom in front of the orifice 11 and a flared margin 15behind and on the side of the orifice 11 that meet the sides of thefront end 13 where they form two edges 17. These edges are sufficientlyrounded to avoid creating zones of weakness (see FIG. 5).

The presence of the rear margin 15 enables the diffusion portion 9 to bemade with a certain amount of tolerance relative to the hole 7. This isshown in FIG. 4 where dashed lines show various positions that theorifice 11 could occupy relative to the diffusion portion 9. As can beseen, in all the examples shown, the orifice 11 opens out completelyinto the diffusion portion 9 at a position such that the cool air streamis guided by the diffusion portion 9, thereby guaranteeing propercooling of the outside surface 5. Naturally, better diffusion isobtained when the orifice 11 opens out substantially into the bottom ofthe diffusion portion 9, as shown in continuous lines.

What is claimed is:
 1. An electrode for forming an indentation in a wallby electroerosion, the electrode comprising: a body; and a tip which isin the form of a cone having a cone end, a conical surface, and a coneaxis, the conical surface including a flat extending on one side of thecone from a point in a vicinity of the cone end beyond a portion of thecone having a maximum flare of the cone, wherein the cone end is roundedabout a circumference of the tip except for the flat of the conicalsurface, and wherein the flat does not intersect the cone axis.
 2. Theelectrode according to claim 1, wherein the cone is symmetric about aplane of symmetry perpendicular to the flat and containing the coneaxis, and a half-angle of the maximum flare defined between side edgesof the flat and the plane of symmetry is between 10° and 30°.
 3. Theelectrode according to claim 2, wherein the half-angle of the maximumflare defined between side edges of the flat and the plane of symmetryis 15°.